Formalize the BehaviorChain.Title().GetHashCode()

[jeremydmiller]

Too much repetition in the code where we use BehaviorChain.Title().GetHashCode() to uniquely refer to a chain. Also add BehaviorGraph.FindChain(identifier)

• BehaviorChain.Key
• BehaviorGraph.FindChain(int key)
• Replace all usages of BehaviorChain.Title().GetHashCode()